1. Technical Field
This invention relates generally to sparkplugs and other ignition devices, and more particularly to electrodes having firing tips on sparkplugs and other ignition devices used in internal combustion engines and there method of construction.
2. Related Art
Within the field of sparkplugs, there exists a continuing need to improve the erosion resistance and reduce the sparking voltage at the sparkplug's center and ground electrode, or in the case of multi-electrode designs, the ground electrodes. Various designs have been proposed using noble metal electrodes or, more commonly, noble metal firing tips applied to standard metal electrodes. Typically, the firing tip is formed as a pad or rivet which is then welded onto the end of the electrode.
Platinum and iridium alloys are two of the noble metals most commonly used for these firing tips. See, for example, U.S. Pat. No. 4,540,910 to Kondo et al. which discloses a center electrode firing tip made from 70 to 90 wt % platinum and 30 to 10 wt % iridium. As mentioned in that patent, platinum-tungsten alloys have also been used for these firing tips. Such a platinum-tungsten alloy is also disclosed in U.S. Pat. No. 6,045,424 to Chang et al., which further discloses the construction of firing tips using platinum-rhodium alloys and platinum-iridium-tungsten alloys.
Apart from these basic noble metal alloys, oxide dispersion strengthened alloys have also been proposed which utilize combinations of the above-noted metals with varying amounts of different rare earth metal oxides. See, for example, U.S. Pat. No. 4,081,710 to Heywood et al. In this regard, several specific platinum and iridium-based alloys have been suggested which utilize yttrium oxide (Y2O3). In particular, U.S. Pat. No. 5,456,624 to Moore et al. discloses a firing tip made from a platinum alloy containing <2% yttrium oxide. U.S. Pat. No. 5,990,602 to Katoh et al. discloses a platinum-iridium alloy containing between 0.01 and 2% yttrium oxide. U.S. Pat. No. 5,461,275 to Oshima discloses an iridium alloy that includes between 5 and 15% yttrium oxide. While the yttrium oxide has historically been included in small amounts (e.g., <2%) to improve the strength and/or stability of the resultant alloy, the Oshima patent discloses that, by using yttrium oxide with iridium at >5% by volume, the sparking voltage can be reduced.
Further, as disclosed in U.S. Pat. No. 6,412,465 B1 to Lykowski et al., it has been determined that reduced erosion and lowered sparking voltages can be achieved at much lower percentages of yttrium oxide than are disclosed in the Oshima patent by incorporating the yttrium oxide into an alloy of tungsten and platinum. The Lykowski patent discloses an ignition device having both a ground and center electrode, wherein at least one of the electrodes includes a firing tip formed from an alloy containing platinum, tungsten, and yttrium oxide. Preferably, the alloy is formed from a combination of 91.7%-97.99% platinum, 2%-8% tungsten, and 0.01%-0.3% yttrium, by weight, and in an even more preferred construction, 95.68%-96.12% platinum, 3.8%-4.2% tungsten, and 0.08%-0.12% yttrium. The firing tip can take the form of a pad, rivet, ball, or other shape and can be welded in place on the electrode.
While these and various other noble metal systems typically provide acceptable sparkplug performance, some well-known and inherent performance limitations associated with the methods which are used to attach the noble metal firing tips to the electrodes, particularly various forms of welding, exist. In particular, cyclic thermal stresses in the operating environments of the sparkplugs, such as those resulting from a mismatch in thermal expansion coefficients between the noble metals and noble metal alloys mentioned above, which are used for the firing tips, and the Ni, Ni alloy and other well-known metals which are used for the electrodes, are known to result in cracking, thermal fatigue and various other interaction phenomena that can result in the failure of the welds, and ultimately of the sparkplugs themselves.